The hill start assist system (HSA) is known as a system for preventing the vehicle from rolling back under its own weight when starting the vehicle on an uphill incline by maintaining the braking force of the vehicle by operating the brake devices of the vehicle when certain conditions are met. See Patent Document 1, for instance.
As illustrated in FIG. 9, the HSA performs it function by being activated (or by maintaining the braking action) when starting forward on an uphill incline or starting rearward on a downhill incline. The inclination of the road surface is typically detected by using a fore and aft G sensor. The intension of the vehicle operator to start forward or rearward can be detected by the shift lever position (in particular, the state of a reverse switch indicating a rearward movement or otherwise) in the case of a manual transmission vehicle (MT vehicle), and the shift lever position determined by the state of shift lever position switches or the selected gear as determined by the control state of the automatic transmission system in the case of an automatic transmission vehicle (AT vehicle). In the case of a belt driven continuous transmission vehicle (CVT vehicle), the intension of the vehicle operator can be detected from the shift lever position as determined by shift lever position switches. When the vehicle starts on an inclined road surface of an inclination angle greater than a prescribed threshold value, the HSA prevents the rolling back of the vehicle by maintaining a brake pressure corresponding to the inclination angle of the road surface.
The shift position is typically determined by the output signals of a number of switches that are turned on and off depending on the presence and absence of the shift lever in the corresponding shift positions. In particular, in the case of an AT or CVT vehicle, a switch is provided in each of a number of shift positions such as reverse (R), neutral (N) and drive (D) positions. Normally, these switches are configured such that any two adjoining switches are not turned on simultaneously.
However, in reality, when the shift lever is positioned intermediate between two different shift positions or any one of the switches has seized in the turned on or turned off state, the two shift positions may be simultaneously detected or no shift position may be detected at times. In such a case, the intention of the vehicle operator (to start forward or rearward) may not be determined To meet such a contingency (when the shift positions are doubly detected or no shift position is detected owing to the failure in the shift position switches), the conventional HSA was incorporated with a control routine as discussed in the following. As illustrated in FIG. 10, in the case of an AT vehicle, when two shift positions are detected at the same time, and one of them consists of the P position, the shift position is determined as the P position. When two shift positions are detected at the same time, and neither of them consists of the P position or when no shift position is detected, the shift position is determined from the gear stage as detected from the control system of the automatic transmission. The lower speed forward shift positions (such as D4, D3, S and L) are omitted from the table in FIG. 10 as they are treated in the same way as the D position. The HSA for CVT vehicles places priorities on the P, R, N and D positions in that order. As illustrated in FIG. 11, when two shift positions are detected at the same time, the one with a higher priority is detected. When no shift position is detected, the P position is detected. The lower speed forward shift positions (such as S and L) are omitted from the table in FIG. 11 as they are treated in the same way as the D position.